The high pressure feeder, or transfer device, is one of the most basic and important components of the Kamyr continuous pulping system. The high pressure feeder is used to transfer steamed wood chips in a liquid (typically white liquor) at low pressure to the top of the continuous digester, at high pressure. A typical high pressure transfer device comprises a pocketed rotor, a housing, a screen, and pump separably connected to the housing. The pocketed rotor has a plurality of through going pockets, each having opposite end openings which function as both inlets and outlets depending upon the angular position of the rotor. The housing encloses the rotor and has an exterior periphery with first through fourth ports disposed around the exterior periphery for registry with the inlets to and outlets from the pockets. The first and third ports are opposite, and the second and fourth ports are opposite, and the first and second ports may be adjacent in the direction of rotation.
In a conventional high pressure feeder screen means are disposed in the third port for screening particles above a predetermined size out of the liquid passing through the third port, and a low pressure pump is connected to the third port to provide the suction for sucking liquid through the third port. A high pressure pump is operatively connected to the second port to provide the flow of liquid under high pressure through the second port. Normally the first port is on the top, and the third port on the bottom, the first port connected to the chips chute, and the fourth port connected to the top of the digester.
While conventional high pressure feeders have functioned very well over the decades they have been in use, there have been relatively few substantive changes to the high pressure feeder over the decades. It has been known that the filling efficiency of the high pressure feeder is approximately 50 to 60% on some chip furnishes, and that is significantly lower than is desired, but to date no significant inroads have been made in substantially increasing that efficiency.
To a large extent, the efficiency of the high pressure feeder is dictated by its ability to obtain the chip chute circulation which carries the chips from the chute into the pockets of the rotor. The chip chute circulation is throttled on the suction side of the chip chute circulation pump by the pressure drop across the screen at the third port. This is due primarily to the pressure drop across blinding material (usually fines and/or debris) on top of the screen, and the pressure drop through the chips in the pocket and any losses in the chute. The chip chute flow is highly cyclical, rising to a maximum almost instantly as a new pocket of the rotor turns into operative association with the chute at the housing first port, and falling as the blinding material on top of the screen and the pressure drop through the chips in the pocket develop.
The maximum chip chute flow as the pocket rotates into operative association with the first port is counterproductive. As the empty pocket comes into operative association with the chip chute, the chips are initially unable to flow through the long narrow slot that is exposed. Thus, fines will flow through the slot, entering the pocket before the larger chips, and any material in the pocket will flow to the screen creating blinding of the screen with its associated pressure drop. Thus, the high flow of the clean screen is wasted.
Another part of the lack of efficiency of the high pressure feeder is due to problems in emptying the pocket as it turns into operative association with the second and fourth ports, to receive the high pressure liquid from the high pressure pump which flushes the chips out of the pocket. The natural construction of the rotor is that the pockets are tapered to a minimum dimension approximately one-half way through the rotor diameter. As a pocket turns into operative association with the second port, the force of the high pressure liquid which instantly enters the pocket tends to compress the chips since in the initial stages of the rotation the slot opening of the pocket outlet into the fourth port is too small to effectively let the chips pass through. This compression of the chips makes it more difficult to release them from the pocket.
According to the present invention, both of the above mentioned problems are alleviated by providing simple modifications of the screen and the second port from what is conventional.
According to the present invention, the high pressure feeder is modified to allow significant flow of particles above a predetermined size through the first port into the pocket inlet before that pocket is operatively exposed to the suction of the chip chute recirculation pulp at the third port. This is preferably and simply accomplished by modifying the conventional screen so that it is blanked at the leading edge thereof in the direction of rotation of the pocket. An arcuate extent of the blanked portion of the screen--compared to what is conventional--is preferably about two inches, but will vary with feeder size. Thus, there is an approximately two inch wide inlet area of the pocket that is exposed to the first port (chip chute) before any part of the outlet of the pocket is exposed to the suction of the recirculation pump. This prevents an initial high velocity flow from drawing fines or debris into the pocket or blinding it before any chips can flow into the pocket; some chips will have already passed into the pocket before the pocket outlet is exposed to the suction source, thereby resulting in maximum utility of the initial high surge of suction, and thereby significantly enhancing the filling efficiency of the high pressure feeder.
According to another aspect of the present invention the emptying efficiency of the high pressure feeder is enhanced, again by a simple modification. A second port is constructed so that in its leading edge in the direction of rotation of the rotor a pre-pressurizing wedge is provided that has an arcuate extent, in the direction of rotation, about two to three inches greater than in conventional feeders. This means that a pocket inlet is not operatively exposed to high pressure liquid passing through the second port from the high pressure pump until that pocket outlet is already in communication with the fourth port over an arcuate extent larger than the largest practical dimension of particles in the pocket. That is, there is a slot between the pocket outlet and the fourth port of about two to three inches in width before the pocket inlet is exposed to the high pressure liquid. This prevents the high pressure flow from compressing the chips into the pocket before the chips can escape through the pocket outlet into the fourth port, and allows maximum utility of the high pressure flush that occurs when the pocket inlet is rotated into operative communication with the second port, thereby significantly increasing the emptying efficiency of the high pressure feeder.
The invention--while not exclusively limited to use with wood chips in the continuous digesting of chips to produce paper pulp (e.g. it can be used with a coal slurry)--is preferably employed in a method of transferring wood chips through the transfer device to boost the flow rate thereof. According to one aspect of the present invention, the method comprises the steps of continuously: (a) Rotating the rotor in the first direction about its axis of rotation; (b) feeding chips in liquid to the first port; (c) applying suction to the third port; (d) screening liquid passing from a pocket through the third port to remove particles of said first size or larger from the liquid so that the particles remain in the pocket and do not pass through the third port; (e) supplying liquid under high pressure to the second port, so that when a pocket is in communication with the second and fourth ports the high pressure liquid forces the particles in the pocket out of the pocket and through the fourth port; and (f) allowing significant flow of particles above said first size through the first port into a pocket inlet before that pocket is operatively exposed to the suction at the third port so that the flow of liquid is prevented from drawing fines or debris into the pocket before the pocket inlet is open enough to allow passage of chips into the pocket. Step (f) is preferably practiced by providing an arcuate extent of about two inches, in the first direction, of communication between the first port and the pocket inlet before the outlet of that pocket is exposed to the suction of the third port. There also preferably is a further step of feeding the chips in liquid from the fourth port to the top of a continuous digester.
According to another aspect of the present invention a method is provided which comprises steps (a)-(e) as set forth above and then provides the further step (f) of preventing each pocket inlet from being operatively exposed to high pressure liquid passing through the second port until the pocket already is in communication with the fourth port over an arcuate extent larger than the largest practical dimension of chips in the pocket. Step (f) is preferably practiced to provide arcuate communication in the first direction between a pocket outlet in the fourth port of about two to three inches before that pocket inlet is exposed to high pressure liquid from the second port.
It is the primary object of the present invention to increase the filling and/or emptying efficiencies of conventional high pressure feeders. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.